Universität Stuttgart
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Item Open Access The Fermi energy as common parameter to describe charge compensation mechanisms : a path to Fermi level engineering of oxide electroceramics(2023) Klein, Andreas; Albe, Karsten; Bein, Nicole; Clemens, Oliver; Creutz, Kim Alexander; Erhart, Paul; Frericks, Markus; Ghorbani, Elaheh; Hofmann, Jan Philipp; Huang, Binxiang; Kaiser, Bernhard; Kolb, Ute; Koruza, Jurij; Kübel, Christian; Lohaus, Katharina N. S.; Rödel, Jürgen; Rohrer, Jochen; Rheinheimer, Wolfgang; De Souza, Roger A.; Streibel, Verena; Weidenkaff, Anke; Widenmeyer, Marc; Xu, Bai-Xiang; Zhang, HongbinChemical substitution, which can be iso- or heterovalent, is the primary strategy to tailor material properties. There are various ways how a material can react to substitution. Isovalent substitution changes the density of states while heterovalent substitution, i.e. doping, can induce electronic compensation, ionic compensation, valence changes of cations or anions, or result in the segregation or neutralization of the dopant. While all these can, in principle, occur simultaneously, it is often desirable to select a certain mechanism in order to determine material properties. Being able to predict and control the individual compensation mechanism should therefore be a key target of materials science. This contribution outlines the perspective that this could be achieved by taking the Fermi energy as a common descriptor for the different compensation mechanisms. This generalization becomes possible since the formation enthalpies of the defects involved in the various compensation mechanisms do all depend on the Fermi energy. In order to control material properties, it is then necessary to adjust the formation enthalpies and charge transition levels of the involved defects. Understanding how these depend on material composition will open up a new path for the design of materials by Fermi level engineering.Item Open Access Accelerating ab initio melting property calculations with machine learning : application to the high entropy alloy TaVCrW(2024) Zhu, Li-Fang; Körmann, Fritz; Chen, Qing; Selleby, Malin; Neugebauer, Jörg; Grabowski, BlazejMelting properties are critical for designing novel materials, especially for discovering high-performance, high-melting refractory materials. Experimental measurements of these properties are extremely challenging due to their high melting temperatures. Complementary theoretical predictions are, therefore, indispensable. One of the most accurate approaches for this purpose is the ab initio free-energy approach based on density functional theory (DFT). However, it generally involves expensive thermodynamic integration using ab initio molecular dynamic simulations. The high computational cost makes high-throughput calculations infeasible. Here, we propose a highly efficient DFT-based method aided by a specially designed machine learning potential. As the machine learning potential can closely reproduce the ab initio phase-space distribution, even for multi-component alloys, the costly thermodynamic integration can be fully substituted with more efficient free energy perturbation calculations. The method achieves overall savings of computational resources by 80% compared to current alternatives. We apply the method to the high-entropy alloy TaVCrW and calculate its melting properties, including the melting temperature, entropy and enthalpy of fusion, and volume change at the melting point. Additionally, the heat capacities of solid and liquid TaVCrW are calculated. The results agree reasonably with the CALPHAD extrapolated values.Item Open Access Asymmetric Rh diene catalysis under confinement : isoxazole ring‐contraction in mesoporous solids(2024) Marshall, Max; Dilruba, Zarfishan; Beurer, Ann‐Katrin; Bieck, Kira; Emmerling, Sebastian; Markus, Felix; Vogler, Charlotte; Ziegler, Felix; Fuhrer, Marina; Liu, Sherri S. Y.; Kousik, Shravan R.; Frey, Wolfgang; Traa, Yvonne; Bruckner, Johanna R.; Plietker, Bernd; Buchmeiser, Michael R.; Ludwigs, Sabine; Naumann, Stefan; Atanasova, Petia; Lotsch, Bettina V.; Zens, Anna; Laschat, SabineCovalent immobilization of chiral dienes in mesoporous solids for asymmetric heterogeneous catalysis is highly attractive. In order to study confinement effects in bimolecular vs monomolecular reactions, a series of pseudo‐C2‐symmetrical tetrahydropentalenes was synthesized and immobilized via click reaction on different mesoporous solids (silica, carbon, covalent organic frameworks) and compared with homogeneous conditions. Two types of Rh‐catalyzed reactions were studied: (a) bimolecular nucleophilic 1,2‐additions of phenylboroxine to N‐tosylimine and (b) monomolecular isomerization of isoxazole to 2H‐azirne. Polar support materials performed better than non‐polar ones. Under confinement, bimolecular reactions showed decreased yields, whereas yields in monomolecular reactions were only little affected. Regarding enantioselectivity the opposite trend was observed, i. e. effective enantiocontrol for bimolecular reactions but only little control for monomolecular reactions was found.Item Open Access Chemical ordering and magnetism in face-centered cubic CrCoNi alloy(2024) Ghosh, Sheuly; Ueltzen, Katharina; George, Janine; Neugebauer, Jörg; Körmann, FritzThe impact of magnetism on chemical ordering in face-centered cubic CrCoNi medium entropy alloy is studied by a combination of ab initio simulations, machine learning potentials, and Monte Carlo simulations. Large magnetic energies are revealed for some mixed L12/L10 type ordered configurations, which are rooted in strong nearest-neighbor magnetic exchange interactions and chemical bonding among the constituent elements. There is a delicate interplay between magnetism and stability of MoPt2 and L12/L10 type of order, which may explain opposing experimental and theoretical findings.Item Open Access Ab initio machine-learning unveils strong anharmonicity in non-Arrhenius self-diffusion of tungsten(2025) Zhang, Xi; Divinski, Sergiy V.; Grabowski, BlazejThe knowledge of diffusion mechanisms in materials is crucial for predicting their high-temperature performance and stability, yet accurately capturing the underlying physics like thermal effects remains challenging. In particular, the origin of the experimentally observed non-Arrhenius diffusion behavior has remained elusive, largely due to the lack of effective computational tools. Here we propose an efficient ab initio framework to compute the Gibbs energy of the transition state in vacancy-mediated diffusion including the relevant thermal excitations at the density-functional-theory level. With the aid of a bespoke machine-learning interatomic potential, the temperature-dependent vacancy formation and migration Gibbs energies of the prototype system body-centered cubic (BCC) tungsten are shown to be strongly affected by anharmonicity. This finding explains the physical origin of the experimentally observed non-Arrhenius behavior of tungsten self-diffusion. A remarkable agreement between the calculated and experimental temperature-dependent self-diffusivity and, in particular, its curvature is revealed. The proposed computational framework is robust and broadly applicable, as evidenced by first tests for a hexagonal close-packed (HCP) multicomponent high-entropy alloy. The successful applications underscore the attainability of an accurate ab initio diffusion database.Item Open Access Influence of In‐doping on the structure and electrochemical performance of compositionally complex garnet‐type solid electrolytes(2025) Alsawaf, Alaa; Karkera, Guruprakash; Diemant, Thomas; Kante, Monaha Veerraju; Schneider, Yannik; Velasco, Leonardo; Bhattacharya, Subramshu S.; Stainer, Florian; Wilkening, Martin; Clemens, Oliver; Janek, Jürgen; Hahn, Horst; Botros, MiriamThe electrochemical instability of electrode active materials as well as the flammability of the organic liquid electrolytes in Li‐ion batteries pose challenges for their safety and long cycle life. Solid electrolytes (SEs) that exhibit high ionic conductivity and a wide electrochemical stability window alleviate these challenges. Garnet‐type Li7La3Zr2O12 is a promising SE for next‐generation all‐solid‐state batteries. Herein, samples are prepared via a modified solid‐state reaction of compositionally complex Li6.3+zLa3Zr1.1-z Nb0.8Gd0.1InzO12 under different sintering atmospheres. X-ray diffraction patterns and Raman spectra prove the formation of a cubic garnet structure. Significant morphological changes are detected upon In‐doping and correlated to the ionic conductivity. The total Li‐ion conductivity of the dense pellets reaches 1 mS cm-1, among the highest reported to date, with an activation energy of 0.38 eV for the macroscopic ion transport obtained by impedance spectroscopy and as low as 0.24 eV for local Li‐ion hopping processes determined by 7Li nuclear magnetic resonance spin‐lattice relaxation measurements. The electronic contribution to the conductivity is negligible (10-10 S cm-1) making this compositionally complex SE a suitable candidate for all‐solid‐state battery applications.Item Open Access Towards recycling of all‐solid‐state batteries with argyrodite sulfide electrolytes : insights into electrolyte and electrode degradation in dissolution‐based separation processes(2025) Wissel, Kerstin; Hu, Zian; Wu, Xuebin; Jacob, Martine; Küster, Kathrin; Starke, Ulrich; Clemens, OliverAll‐solid‐state Li‐ion batteries (ASSBs) represent a promising leap forward in battery technology, rapidly advancing in development. Among the various solid electrolytes, argyrodite thiophosphates Li6PS5X (X=Cl, Br, I) stand out due to their high ionic conductivity, structural flexibility, and compatibility with a range of electrode materials, making them ideal candidates for efficient and scalable battery applications. However, despite significant performance advancements, the sustainability and recycling of ASSBs remain underexplored, posing a critical challenge for achieving efficient circular processes. This study investigates the dissolution‐based separation and recovery of argyrodite thiophosphate electrolytes and transition metal oxide electrode materials as a potential recycling strategy for ASSBs. A focus is set on the impact of solvent treatments on the recrystallization behavior of these electrolytes. Furthermore, the interactions between dissolved argyrodite thiophosphates and various transition metal oxide electrode materials (LiCoO2, LiMn2O4, LiNi0.8Mn0.1Co0.1O2, LiFePO4 and Li4Ti5O12) is examined to assess their influence on the functional properties of both the electrolytes and electrode materials. Structural, compositional and morphological changes are analyzed using X‐ray diffraction, scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, inductively coupled plasma mass spectrometry and X‐ray photoelectron spectroscopy. Our findings provide insights into the complexities of recycling ASSBs, but also highlight the potential for developing efficient, sustainable recycling processes.Item Open Access 3D sub-nanometer analysis of glucose in an aqueous solution by cryo-atom probe tomography(2021) Schwarz, T. M.; Dietrich, C. A.; Ott, J.; Weikum, E. M.; Lawitzki, R.; Solodenko, H.; Hadjixenophontos, E.; Gault, B.; Kästner, J.; Schmitz, G.; Stender, P.Atom Probe Tomography (APT) is currently a well-established technique to analyse the composition of solid materials including metals, semiconductors and ceramics with up to near-atomic resolution. Using an aqueous glucose solution, we now extended the technique to frozen solutions. While the mass signals of the common glucose fragments CxHy and CxOyHz overlap with (H2O)nH from water, we achieved stoichiometrically correct values via signal deconvolution. Density functional theory (DFT) calculations were performed to investigate the stability of the detected pyranose fragments. This paper demonstrates APT’s capabilities to achieve sub-nanometre resolution in tracing whole glucose molecules in a frozen solution by using cryogenic workflows. We use a solution of defined concentration to investigate the chemical resolution capabilities as a step toward the measurement of biological molecules. Due to the evaporation of nearly intact glucose molecules, their position within the measured 3D volume of the solution can be determined with sub-nanometre resolution. Our analyses take analytical techniques to a new level, since chemical characterization methods for cryogenically-frozen solutions or biological materials are limited.Item Open Access Free-energy perturbation in the exchange-correlation space accelerated by machine learning : application to silica polymorphs(2025) Forslund, Axel; Jung, Jong Hyun; Ikeda, Yuji; Grabowski, BlazejWe propose a free-energy-perturbation approach accelerated by machine-learning potentials to efficiently compute transition temperatures and entropies for all rungs of Jacob’s ladder. We apply the approach to the dynamically stabilized phases of SiO2, which are characterized by challengingly small transition entropies. All investigated functionals from rungs 1-4 fail to predict an accurate transition temperature by 25-200%. Only by ascending to the fifth rung, within the random phase approximation, an accurate prediction is possible, giving a relative error of 5%. We provide a clear-cut procedure and relevant data to the community for, e.g., developing and evaluating new functionals.Item Open Access Defect-engineered perovskites : atomic scale nature of A‑site vacancy-stabilized catalytically active phase(2026) Talei, Roham; Mohammadi, Asghar; Winterstein, Thomas F.; Malleier, Christoph; Schmitz, Guido; Penner, Simon; Bonmassar, NicolasUsing aberration-corrected electron microscopy and spectroscopy, we reveal the atomic scale structure and catalytic function of the A-site-deficient perovskite La0.7Fe0.7Mn0.3O3, uncovering a heterogeneous defect landscape that governs its activity in reducing NO by CO. We identified a layer of La0.7-xFe0.7Mn0.3O3 that is highly A-site-deficient. This layer is just two to three unit cells thick at the surfaces and the interfaces of the perovskite particles and transitions toward the bulk into stoichiometric LaFe0.7Mn0.3O3. These confined defect layers stabilize catalytically active sites, enabling the formation of FeOx inclusions ranging from approximately 1 nm to several nanometers in size at the surfaces. In situ surface characterization and catalytic measurements reveal that these interfacial FeOx nanoparticles serve as active sites during the NO reduction by CO via the Mars-van Krevelen mechanism. Our findings establish a direct relationship between the structure and properties of nanoscale A-site nonstoichiometry and redox-driven catalytic activity. This relationship offers a new design strategy for tailoring reactivity through defect engineering.